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Alternatives to QFTby waterfall
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#199
Feb2412, 02:22 PM

P: 476

You cite chapter 18 in MTW but that only deals with linearized GR. What linearized GR can be thought as the theory of a massless spin2 field was acknowledged above in one of my posts. 


#200
Feb2412, 02:42 PM

P: 476




#201
Feb2412, 02:48 PM

P: 476

It is possible to derive GR (geometrodynamics) from a field theoretic approach to gravitation, but as an geometric approximation. Somewhat as geometrical optics is an approximation to physical optics based in fields. 


#202
Feb2412, 05:08 PM

P: 381

Someone asked Bill there: > But in string theory, spacetime still has curvature. Bill replied: "No it doesn't. It emerges as a limit  but the underlying geometry of spacetime  if it has one  is not known." Someone asked Bill again: > Are you implying that in string and superstring theory, spacetime is flat and what caused gravity >are gravitons? Bill replied: "It has long been known that a quantum theory of gravity as spin two particles in a flat spacetime leads to GR eg the link I seem to have to give over and over: http://arxiv.org/abs/grqc/9512024 " Bill clearly stated that in string theory, spacetime has no curvature and it is the spin two particles in a flat spacetime that lead to GR! So Bill is clearly talking about String theory and not classical physics. Now since spin2 fields in flat spacetime in classical physics is not completely right. Then how could he bring it to string theory? This is the part I can't understand. Bill, can you clarify this or someone can state once and for all that he has some misunderstanding here (and clarify it), at least to settle the issues because I've been thinking for this for over 5 years already. Or if you still can't understand my point. Just answer this: Does as Bill put it, a "quantum theory of gravity as spin two particles in a flat spacetime leads to GR"?? 


#203
Feb2412, 05:56 PM

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P: 2,474

Lest anyone puts words in my mouth those posts are many years old.
My position is this. Spin 2 particles imply linearised gravity and linearised gravity implies full GR. There may be other issues involved  let see what emerges when people who are into this sort of stuff discuss it. There is something in the back of my mind where I have seen this discussed before and really it didn't lead anywhere. Thanks Bill 


#204
Feb2412, 05:58 PM

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P: 8,328




#205
Feb2412, 06:08 PM

P: 381

In classical physics. This http://www.worldscinet.com/ijmpd/17/...1808012085.pdf shows spin 2 particles in flat spacetime CAN'T lead to GR. How is it that in a quantum theory of gravity like String theory, spin 2 particles in flat spacetime CAN lead to GR while in classical physics, It CAN'T (as juanrga emphased in his shared paper)? 


#206
Feb2412, 06:09 PM

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#207
Feb2412, 06:12 PM

P: 381

"There is more to gravity than gravitons. (There is sufficient evidence to assume that gravity is not a fundamental field but an emergent phenomenon like elasticity." Please read the paper written by India top physicist which disproves that in classical GR, spin2 fields in flat spacetime can lead to GR. If it doesn't apply classically. You can't apply it in quantum gravity classical limit. 


#208
Feb2412, 06:19 PM

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When he says gravity is more than gravitons, he is talking about quantum gravity near the Planck scale  there Padmanabhan favours emergent gravity like string theory: "There is more to gravity than gravitons. (There is sufficient evidence to assume that gravity is not a fundamental field but an emergent phenomenon like elasticity. ..." 


#209
Feb2412, 06:34 PM

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#210
Feb2512, 07:42 AM

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#211
Feb2512, 08:10 AM

P: 381

I believe with a little fixing, the quantum spin2 field would be the primary entity and the geometry merely as a result of the symmetry in the math of the quantum field theory. It's better than believing gravity is only geometry as General Relativity folks love to express. Therefore I'm more inclined now toward string theory especially MTheory which may involve what Witten describes as an incredible quantum symmetry where strings are just temporary constructs or a dual bit. I'm not sure about Loop Quantum Gravity. If it's about geometry and reverse engineering it to get to the quantum parts or spin networks. Then it has less elegance. 


#212
Feb2512, 05:34 PM

P: 381

In quantum field theory, the quanta like photons and gravitons are just momentum and energy of the fields. Of course you need the entire fields to do the work although what you can measure are simply the photons and gravitons but the fields which you take as the ocean underneath the "small ripples on a water surface" need to be active with the properties it needs. Now in the case of our spin2 gravitons discussions. To be sure I understood the concept. Let me explain it to you. What they meant when they say spin2 field over flat spacetime equaled curved spacetime and gravity is not that spin2 gravitons is enough to pull off those trick. But the ocean or gravitational fields derived from a quantum gravity has the properties and right coupling to pull of the gravity act. Then the spin2 gravitons are just manifestation of this hidden gravitational fields. Therefore to avoid confusion. I think the proper things to say is instead of: "Spin 2 gravitons + flat spacetime = General Relativity". One must say this: "Spin 2 gravitons (with underlying gravitational field produced from excitations of strings or LQG or others) + flat spacetime = General Relativity." Are we clear on this now. You guys used the former descriptons all throughout hence you confused even others like "surprised". Right? 


#213
Feb2512, 06:00 PM

P: 381

In post #99 a week ago:
We had a long detour on string theory and spin2 graviton thing because I was asking Marcus above (in post #98) if Loop Quantum Gravity was also about spin2 graviton on flat spacetime and up to now it isn't answered because Marcus reactions to this spin2 graviton idea is the above. Well. So how do spin2 gravitons (plus gravity fields) over flat spacetime explains Big Bang expansion? I guess we can consider the spin2 fields as unique in that the fields can expand. Remember the Inflaton is also a field.. so it fills the gravity fields with inflatons expanding the fields with the effect as like producing spacetime curvature (but not really). Isn't it? About Loop Quantum Gravity. So we can also consider it as spin networks producing the right coupling of gravity and hence can also be consider as having graviton spin2 field over flat spacetime. Meaning spacetime only appears curved in LQG but not really curved?? This was what I was asking Marcus prior to his reply above whether one can consider LQG as like String theory where it is about spin2 gravity fields over flat spacetime with the curvature geometry as not really a priori. But Marcus, like fellow poster surprised, misunderstood the concept as I didn't add the gravity field (behind spin 2) idea so didn't answer it. So let me ask this again now so someone can answer this LQG question above and we can close this thread clean. Thanks. 


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